Thursday, February 21, 2013

How to swap two numbers without using temp or third variable is common
interview question not just on Java interviews but also on C and C++
interviews. It is also a good programming questions for freshers.
This question was asked to me long back and didn't had any idea about how to
approach this question without using temp or third variable, may be lack of knowledge
on bitwise operators in Java or may be it didn't click at that time. Given some
time and trial error, I eventually come out with a solution with just
arithmetic operator but interviewer was keep asking about other approaches of swapping two variables without using temp or
third variable. Personally, I liked this question and included in list of
my programming interview question
because of its simplicity and some logical work, it force you to do. When
learned bit-wise operation in Java I eventually find another way of swapping two
variables without third variable, which I am going to share with you guys.

Swapping two numbers without using temp variable in Java

If you have ever heard this question, then you must be familiar with this
approach of swapping numbers without using temp variable. If you are hearing it
first time, then try it yourself, its a good programming exercise for
absolute first timer. By the way, here is the code example of swapping two
numbers without using temp variable and using arithmetic operator in Java:

int a =10;

int b =20;

System.out.println("value of a and b before swapping, a: "+ a +" b: "+ b);

//swapping value of two numbers without using temp variable

a = a+ b;//now a is 30 and b is 20

b = a -b;//now a is 30 but b is 10 (original value of a)

a = a -b;//now a is 20 and b is 10, numbers are swapped

System.out.println("value of a and b after swapping, a: "+ a +" b: "+ b);

Output:

value of a and b before swapping, a:10 b:20

value of a and b after swapping, a:20 b:10

Swapping two numbers without using temp
variable in Java with bitwise operator

Bitwise operators can also be used to swap two numbers without using
third variable. XOR bitwise operator returns zero if both operand is same i.e.
either 0 or 1 and returns 1 if both
operands are different e.g. one operand is zero and other is one. By leveraging this property, we can swap two numbers in Java. Here is code
example of swapping two numbers without using temp variable in Java using XOR
bitwise operand:

ABA^B (AXORB)

000 (zero because operands are same)

011

101 (one because operands are different)

110

int a =2;//0010 in binary

int b =4;//0100 in binary

System.out.println("value of a and b before swapping, a: "+ a +" b: "+ b);

//swapping value of two numbers without using temp variable and
XOR bitwise operator

a = a^b;//now a is 6 and b is 4

b = a^b;//now a is 6 but b is 2 (original value of a)

a = a^b;//now a is 4 and b is 2, numbers are swapped

System.out.println("value of a and b after swapping using XOR bitwise operation, a: "+ a +" b: "+ b);

value of a and b before swapping, a:2 b:4

value of a and b after swapping using XOR bitwise
operation, a:4 b:2

Swapping two numbers without using temp
variable in Java with division and multiplication

There is another, third way of swapping two numbers without using third
variable, which involves multiplication and division operator. This is similar
to first approach, where we have used + and - operator for swapping values of
two numbers. Here is the code example to swap tow number without using third
variable with division and multiplication operators in Java :

int a =6;

int b =3;

System.out.println("value of a and b before swapping, a: "+ a +" b: "+ b);

//swapping value of two numbers without using temp variable using
multiplication and division

a = a*b;//now a is 18 and b is 3

b = a/b;//now a is 18 but b is 6 (original value of a)

a = a/b;//now a is 3 and b is 6, numbers are swapped

System.out.println("value of a and b after swapping using multiplication and division,
a: "+ a +" b: "+ b);

Output:

value of a and b before swapping, a:6 b:3

value of a and b after swapping using multiplication and division,
a:3 b:6

That's all on 3 ways to swap two variables without using third
variable in Java. Its good to know multiple ways of swapping two variables
without using temp or third variable to handle any follow-up question. Swapping
numbers using bitwise operator is the fastest among three, because it involves
bitwise operation. It’s also great way to show your knowledge of bitwise
operator in Java and impress interviewer, which then may ask some question on
bitwise operation. A nice trick to drive interview on your expert area.

"Are there any real world examples where somebody would want to use any of these solutions? "

No, which is why it is a stupid interview question and if your interviewer asks it, you might think twice about taking the job there. Remember an interview is where you decide if you want to work there in addition to the company deciding if they want to hire you.

@Anonymous, In real world, I suggest to use temp variable. It's easy, readable and works perfectly.In my opinion, these kind of questions has there place, they make you to think and put your thought on code, point out edge cases, limitations etc. Don't you desire all those quality in anyone joining your team?

@Anonymous, you can safely use 1st and 2nd code sample to swap two numbers in Java, they are free from Integer overflow. But, don't use this kind of swapping code to actually swap two values, It's not worth. In all cases temp variable should be used to swap numbers including integers, double or any floating point numbers.

From a performance point of view, there is not much merit in doing a swap without using a third variable. If you study the bytecode that's generated for each of the methods, you will probably find that using a temporary variable generates the least number of bytecode instructions (6 as opposed to 12 or more). So, if you have something like this within a tight loop, use a third variable. While it's a cool idea, be sure to mention the downside to your interviewer.

If some interviewer asks me this question I will quit that interview and leave. This is so unreal and so academic if you are not hiring for some very specialised position, not to mention all that problems with overflow, performance etc...

My brain is activelly refusing to waste precious time with problems like this and from my > 10 years programming experience this type of question will tell you nothing about your candidate and his programming skills and experience. And he is often to nervous to solve algorhitmical problems when sweatting in front of interviewer, if he will come up with absoluttely nothing, it doesnt mean he is very smart.

Maybe that only thing you will learn if he will solve that right on place is that he is reading this blog regurally :)